This invention relates generally to the art of electrochromic cells, and more particularly relates to the art of transparent electrochromic devices utilizing iridium oxide as a complementary electrochromic film and as the charge-balancing counter electrode.
Conventional electrochromic cells comprise a thin film of a persistent electrochromic material, i.e., a material responsive to the application of an electric field of a given polarity and sufficient voltage to change from a high-transmittance, non-absorbing state to a lower-transmittance, absorbing or reflecting state. The film of electrochromic material remains substantially in the lower-transmittance state after the electric field is discontinued. When an electric field of opposite polarity is applied to the electrochromic material, it returns to the prior high-transmittance state. The film of electrochromic material, which is both an ionic and electronic conductor, is in ion-conductive contact, preferably direct physical contact, with a layer of ion-conductive material. The ion-conductive material may be a solid, liquid or gel. The electrochromic film and ion-conductive layer are disposed between two electrodes to form a cell. In some applications, a complementary electrochromic film is used to form a cell, while in other applications an optically passive film or metal is used in place of the complementary electrochromic film to form the cell.
As voltage is applied across the two electrodes, ions are conducted through the ion-conducting layer. When the electrode adjacent to the film of electrochromic material is the cathode, and the electrochromic material is cathodically coloring, application of an electric field to that material causes darkening of the film. Reversing the polarity of the electric field causes reversal of the electrochromic properties, and the film reverts to its high transmittance state. Conventionally, a cathodically coloring electrochromic material, usually tungsten oxide or compounds thereof, is deposited on a glass substrate that has been previously coated with an electroconductive film such as tin oxide or indium/tin oxide (ITO) to form one electrode. In some electrochromic devices, the counter electrode has been a carbon-paper structure backed by a similar tin oxide or indium/tin oxide coated glass substrate or metal plate.
Schiavone describes the deposition of electrochromic iridium oxide by reactive sputtering of an iridium target in the Journal of the Electrochemical Society, Vol. 128 No. 6 pp. 1212-1214 (June 1981). Iridium oxide films are deposited by reactive radio frequency sputtering of an iridium target in pure oxygen to deposit a film in the fully colored state.
Kang and Shay describe blue sputtered iridium oxide films in the Journal of the Electrochemical Society, Vol. 130, No. 4, pp. 766 et seq. (April 1983). The properties are compared with those of black sputtered iridium oxide films and the properties of anodically grown iridium oxide films. The blue iridium oxide films are deposited by reactive direct current sputtering of iridium in a 80/20 argon/oxygen gas mixture onto glass substrates held at 238.degree. K. The process used by the authors include target pre-sputtering for 15 minutes in pure argon followed by 25 minutes of in-situ substrate cleaning and another pre-sputtering step for 10 minutes in the argon/oxygen gas mixture used for the actual coating. As electrochromic display electrodes, the blue sputtered iridium oxide films are described as having an improved open-circuit bleached state and written-state memory, as well as an improved appearance.
Cogan et al describe electrochromic devices in SPIE, Vol. 823, No. 482 et seq. (1987) comprising indium/tin oxide (ITO) coated glass sheets, electrochromic films of tungsten oxide and iridium oxide, and a polymer electrolyte of poly-2-acrylamido-2-methylpropane-sulfonic acid (polyAMPS).
U.S. Pat. No. 4,350,414 to Takahashi et al discloses a solid state electrochromic device comprising a pair of electrodes, an oxidizable film of iridium hydroxide and/or nickel hydroxide, a reducible film of tungsten oxide and/or molybdenum oxide, and an insulating film between said films that allows proton conduction, but prohibits electron conduction, e.g., tantalum oxide, zirconium oxide, niobium oxide, alumina, magnesium fluoride, silicon oxide, titanium oxide, hafnium oxide or yttrium oxide.
U.S. Pat. No. 5,327,281 to Cogan et al describes a solid polymer electrolyte for an electrochromic device which is laminated between an amorphous tungsten trioxide coated indium/tin oxide coated glass substrate and an iridium oxide coated indium/tin oxide coated glass substrate.